A growing number of travelers are signing up for the government's expedited airport screening programs, only to face another wait.

After angry fliers missed flights this spring because of long security lines, government officials promoted the PreCheck and Global Entry systems. The number of applicants for PreCheck more than tripled in months, climbing to 16,000 a day in May.

Now there is a new logjam. It can take weeks or even months to get an appointment for a brief in-person interview to complete the enrollment. Travelers can try walking into an enrollment office without an appointment, but that can mean waiting for hours or even getting turned away.

The Transportation Security Administration recently improved wait times at U.S. airports by hiring more agents and paying more overtime. But any hiccups in signing up travelers for expedited screening could slow down the government's efforts to revamp airport security and foil some travelers' hopes of speeding through security this summer.

Enrolling in either PreCheck or Global Entry allows fliers to use the expedited screening lanes at major U.S. airports. Members can keep shoes and belts on, keep liquids and laptops in their bags and walk through regular metal detectors instead of full-body scanners. The TSA can process 150 passengers an hour in a standard lane, 300 an hour in a PreCheck lane.

But many city-center PreCheck enrollment facilities don't have any available appointments for the next 45 days, the most their schedule allows.

Those walking in without an appointment should arrive early and be prepared to wait. An Associated Press reporter recently showed up at a Dallas PreCheck center at 8:15 a.m. There were already 18 people crowded into the small waiting room, taking up all the available seats. One employee was processing all the applicants.

By 11 a.m., only people with appointments were being accepted. By noon, the wait was so bad one traveler had a pizza delivered. Ultimately, the reporter waited more than six hours — until 2:30 p.m. — to be called. The application interview itself took just minutes.

In other parts of the country, the waits are minimal.

The people in Dallas could have been seen immediately — if they were willing to drive 177 miles to an enrollment center in Lawton, Okla.

An AP reporter got an appointment in Belleville, Mich. — between Ann Arbor and Detroit — just one day after applying online. The hardest part: finding the center inside an H&R Block tax office, sandwiched between a check-cashing place and a pharmacy in a rundown strip mall.

'It is frustrating,' acknowledges John Sammon, chief marketing officer for the TSA. He advises travelers to 'explore the regional areas and see what's available.'

In the meantime, the TSA works to add more capacity. MorphoTrust USA, the private vendor that handles all PreCheck applications at its IdentoGO Centers, is moving staff from underutilized spots to overcrowded urban ones.

The TSA also seeks bids for a faster way to enroll travelers, using existing databases to verify identity, citizenship and any criminal record. That new process, Sammon says, may or may not require an interview.

The agency hopes to ultimately have half the fliers passing through checkpoints on any given day to be in a so-called trusted traveler program. To hit that goal, the agency estimates it needs 25 million frequent travelers enrolled.

Membership in both PreCheck and Global Entry is valid for five years. PreCheck costs $85. Global Entry costs $15 more and also allows fliers to skip immigration lines when re-entering the country. That's why it tends to be more popular with road warriors.